Re: [Discuss-gnuradio] USRP spike

[Brian Padalino]

On Thu, Aug 12, 2010 at 1:56 PM, naveen nischal
<address@hidden> wrote:
>
> > Hi All,
> >
> > We have been using the USRP1 with the WBX card on it to communicate with
> > the AO-51 satellite. We are expecting to hear from the satellite using
> > an Fm Receiver GRC at 435.300 MHz but don't receive anything as yet, we
> > are pretty sure about our antenna setup and the GRC are right. The
> > problem though is a spurious spike of about 17db which appears at
> > whatever center frequency we tune to in the spectrum. we think that this
> > might be the problem as that might be jamming the signal which was
> > supposed be about the same db level. The point of notice for us is that
> > this spike is always there even without the antenna connected. the fft
> > is directly the output of the usrp source in the grc so, we are
> > presuming that the problem might be in the usrp motherboard .
> > Attached is the screenshots FM receiver spectrum. Did anyone have this
> > problem? How can we fix it?.

Terminate your antenna input.  Does it still show up?

Chances are you have a DC offset at the ADC that needs to be removed.
This will happen if the DDC in the FPGA isn't required to resolve any
frequency offset due to the limitations of the LO in the RF chain.

One way to mitigate this is to tune a little bit away from your signal
of interest, then mix your signal of interest to baseband, and filter
off the DC component.

> Thanks,
> Naveen

Hope this helps, and good luck.

Brian